C# XML file remove duplicated and sum values with LINQ

See more:

C#

XML

LINQ

Basically, I'm removing the duplicated entries and summing their values to have only one of each entry.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;

namespace ConsoleApplication2
{
    class Program
    {
        //File
        const string FILE = "ccc.xml";
        static void Main(string[] args)
        {
            XDocument doc = XDocument.Load(FILE);

            List <XElement> originalInvoices = doc.Descendants("Invoice").ToList();

            var groups = originalInvoices.GroupBy(x => (string)x.Element("Hash")).ToList();

            var finalInvoices = groups.Select(x => new {
                unit = x.Descendants("UnitPrice").Sum(z => (decimal)z),
                credit = x.Descendants("CreditAmount").Sum(z => (decimal)z),
                tax = x.Descendants("TaxPayable").Sum(z => (decimal)z),
                net = x.Descendants("NetTotal").Sum(z => (decimal)z),
                gross = x.Descendants("GrossTotal").Sum(z => (decimal)z),
                first = x.First()
            }).ToList();

            foreach (var finalInvoice in finalInvoices)
            {
                finalInvoice.first.Element("Line").SetElementValue("UnitPrice", finalInvoice.unit);
                finalInvoice.first.Element("Line").SetElementValue("CreditAmount", finalInvoice.credit);
                finalInvoice.first.Element("DocumentTotals").SetElementValue("TaxPayable", finalInvoice.tax);
                finalInvoice.first.Element("DocumentTotals").SetElementValue("NetTotal", finalInvoice.net);
                finalInvoice.first.Element("DocumentTotals").SetElementValue("GrossTotal", finalInvoice.gross);
            }

            doc.Element("SalesInvoices").ReplaceWith(new XElement("SalesInvoices", finalInvoices.Select(x => x.first)));
            Console.WriteLine(doc);
            Console.ReadKey();

        }
    }
}

The problem is that my code is exploding when it reaches the following line at the end:

`doc.Element("SalesInvoices").ReplaceWith(new XElement("SalesInvoices", finalInvoices.Select(x => x.first)));`

With the following error: `An unhandled exception of type 'System.NullReferenceException' occurred in ConsoleApplication2.exe`

I can't fix it my file is this one: [XML] <?xml version="1.0" encoding="UTF-8"?> <AuditFile xmlns="urn:OECD:StandardAudit - Pastebin.com[^]

Basically, the code should delete the Invoices repeated Hash and sum the values of UnitPrice, CreditAmount, TaxPayable, NetTotal, and GrossTotal, where UnitPrice and Credit must be the same as NetTotal.

So instead of having many Invoices with the same Hash, I will have only one Invoice with that hash that has the sum of the values above.

The code already removes the duplicated ones and sums the values but somehow I don't know how to fix the problem.

It would be great if someone helps me with the solution for this problem, and I also need to save the "edited" file, which is not happening but my main problem is getting rid of that error.

What I have tried:

//File
            const string FILE = "ccc.xml";
            static void Main(string[] args)
            {
                XDocument doc = XDocument.Load(FILE);
    
                List <XElement> originalInvoices = doc.Descendants("Invoice").ToList();
    
                var groups = originalInvoices.GroupBy(x => (string)x.Element("Hash")).ToList();
    
                var finalInvoices = groups.Select(x => new {
                    unit = x.Descendants("UnitPrice").Sum(z => (decimal)z),
                    credit = x.Descendants("CreditAmount").Sum(z => (decimal)z),
                    tax = x.Descendants("TaxPayable").Sum(z => (decimal)z),
                    net = x.Descendants("NetTotal").Sum(z => (decimal)z),
                    gross = x.Descendants("GrossTotal").Sum(z => (decimal)z),
                    first = x.First()
                }).ToList();
    
                foreach (var finalInvoice in finalInvoices)
                {
                    finalInvoice.first.Element("Line").SetElementValue("UnitPrice", finalInvoice.unit);
                    finalInvoice.first.Element("Line").SetElementValue("CreditAmount", finalInvoice.credit);
                    finalInvoice.first.Element("DocumentTotals").SetElementValue("TaxPayable", finalInvoice.tax);
                    finalInvoice.first.Element("DocumentTotals").SetElementValue("NetTotal", finalInvoice.net);
                    finalInvoice.first.Element("DocumentTotals").SetElementValue("GrossTotal", finalInvoice.gross);
                }
    
                doc.Element("SalesInvoices").ReplaceWith(new XElement("SalesInvoices", finalInvoices.Select(x => x.first)));
                Console.WriteLine(doc);
                Console.ReadKey();
    
            }
        }
    }